Tuning forks



Feb. 1-0, 1970 c. G. RAWLINGS 3,494,122

TUNING FORKS Filed June 6, 1967 2 Sheets-Sheet 1 14 15 'SI 16 21 PR/OEART 3 17 If 3 f 7 Tm CHAN y Rnwuuos A United States Patent TUNING FORKS Charles Guy Rawlings, Newbridge Works, Bath, England Filed June 6, 1967, Ser. No. 643,944

Claims priority, application Great Britain, July 7, 1966,

30,483/ 66 Int. Cl. G04c 3/00 vs. C]. 58-23 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to tuning forks, and more particularly to tuning forks which are intended for use in electromechanical oscillators.

Tuning forks which are electrically maintained in oscillation are being increasingly used for controlling the operation of clocks and other timing mechanisms, and it is important that tuning forks for these purposes should be very precise in operation and producible at low cost. The principal object of the invention is to provide a form of tuning fork which fulfills these requirements.

The conventional tuning fork which is generally used as a tone standard has a stem at the junction of the two tines and it is often machined from a solid block of metal, which is an expensive operation. There is a point at the junction of the two tines and the stem, the nodal point or neutral axis, at which no vibration takes place and the fibres of the material of which the fork is made are completely unstressed at this axis during oscillation of the tines. The section of the fork at this point is comparatively thick and heavy, and therefore very rigid.

For use in an electromechanical oscillator the fork should be as simple and as light in weight as possible, and it must also be producible at low cost. It is therefore usual to make the fork by bending a strip of metal into the approporiate U-shape and to attach a support at the nodal point at which there is no vibration, that is, the centre of the curved portion. However, it is desirable to strengthen the section of the fork at this pointto increase its rigidity and this is provided for by the invention.

In one aspect the invention consists of a method of making a tuning fork comprising the'steps of bending a length of strip material to a U-shape, and deforming the centre of the curved portion of the fork in order to increase its rigidity.

In another aspect the invention consists of a tuning fork comprising a strip of metal bent to a U-shape, the metal strip being distorted at the centre of the curved portion of the -U to increase its rigidity. There may be a support member attached to the fork at the distorted region.

In one form of tuning fork according to the invention the U portion is deformed by bending the edges of the strip outwardly to form a generally channel-shaped region, and preferably this region conforms to the shape of the support member, which may be welded, soldered, riveted or otherwise fixed in position on the fork.

In another form of the invention the two edges of the strip are bent inwardly, leaving the centre portion of the strip projecting outwardly in relation thereto, and the 3,494,122 Patented Feb. 10, 1970 support member is fixed to the inside of the bend in the U-portion.

Preferably the deformation is so contrived that the support may be fixed to the fork at a neutral region in which there is no stress in the material when the fork is in operation.

The strip material of which the fork is made is preferably of the kind which has a zero temperature coefficient of elasticity.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which- FIGURE 1, marked prior art shows a tuning fork of conventional type to assist explanation;

FIGURE 2 is a sideelevation of the curved portion of a tuning fork made from a strip of material bent into a U;

FIGURE 3 is a side elevation of a tuning fork made according to the invention;

FIGURE 4 is an inverted plan view of the tuning fork of FIGURE 3 looking in the direction of the arrow IV in FIGURE 3;

FIGURE 5 is a sectional elevation of the tuning fork of FIGURE 3 taken on the line V-V in FIGURE 3;

FIGURE 6 is a transverse section through the centre of the curved portion of a tuning fork showing the neutral region;

FIGURE 7 shows this strip after deforming in order to produce local strengthening of the section;

FIGURE 8 shows the section of FIGURE 6 bent to a much smaller radius; and

FIGURE 9 is a pictorial view of a part of a fork formed as shown in FIGURE 8.

The conventional tuning fork is machined from a solid piece of material, as illustrated in FIGURE 1, to provide two tines, respectively 11 and 12, which are in one piece with a stem 13 by which the fork may be mounted and supported. Due to its shape the portion of the fork in the centre of the U, indicated by reference 14, is rigid and the two tines vibrate in anti-phase and at the same frequency.

There is a point, approximately indicated at 21, at which no oscillation takes place when the tines of the fork are oscillating. This is the nodal point.

The invention is concerned with tuning forks which are made by bending a flat strip of material into the required U-form. FIGURE 2 shows the curved portion of the U and indicates the nodal point 22 at which there is no oscillation. When the fork is vibrating the two tines are alternately moving towards and away from each other. During the part of the oscillation cycle when the two tines are moving apart the material forming the inner radius of the U, indicated by the line 23, is in tension while the material forming the outer radius, indicated by the line 24, is in compression. On the other hand, the material in the centre region of the fork, indicated by the dotted line 25, is completely unstressed and this is a neutral region. When the tines are moving in the 0pposite direction the material in the surface 23 is in compression while the material in the surface 24 is in tension, the neutral region being again along the line 25.

According to the invention a tuning fork is made from a length of flat strip material which is initially bent into the required U form to provide the tines, respectively 15 and 16 in FIGURE 3, which are joined by the curved centre portion of the U. In order to provide a degree of rigidity at the centre of the bend in the U comparable with that provided in the tuning fork shown in FIGURE 1 the material at the centre of the U, generally indicated at 17, is deformed and in one embodiment of the invention that is conveniently carried out by bending the edge portion 18 and 19 outwardly so as to provide a region which is generally of channel shape, as shown in FIG- URES 3, 4 and 5.

Advantageously this shape is made such as to conform with the shape of a support member to which the fork is to be attached. Such a support member is indicated by dotted lines 20 in FIGURE 5, which shows that the deformed portion of the fork fits closely around the support member and may readily be fixed thereto by soldering, brazing, spot welding, riveting or any other convenient means of attachment.

Advantageously the strip material used for making the fork is a nickel iron alloy of the kind having a substantially zero temperature coefficient of elasticity, such as the materials known as Ni-Span C and Ni-Span D.

FIGURE 6 is a cross-section through a strip of material which may be bent to the U-shape required for the manufacture of a fork according to the invention. The typical section shown has a width of three units and a thickness of one unit. If such a section at the centre of the curved portion of the U is bent to a radius as shown in FIGURES 7 in order to increase its rigidity the neutral band at which the fibres of the metal are unstressed lies along a dotted line 26 in FIGURE 7. On the other hand if the same section is bent around a much smaller radius the line of unstressed fibres is on the dotted line 27 in FIGURE 8. In the case illustrated in FIGURE 7 the radius of curvature has been so chosen that the line 26 is tangential to the curve 28 representing the inner radius to which the strip has been bent. In the case illustrated in FIGURE 8 the radius of curvature 29 has also been chosen so that the line 27 is tangential to the curve 29 to which the strip has been deformed. FIGURE 9 shows a fork in which the centre portion of the U is bent inwardly around the radius 29 as shown in FIGURE 8 and a support 32 is attached at the centre of this radius, so that the joint between the fork and the support is not stressed. In consequence the support and the joint do not change their rigidity, and hence the frequency of the fork is not affected by the support, even after years of operation.

It can be shown that the moment of inertia of the crosssection shown in FIGURE 9 against bending during fork vibration has been increased by 1.9 or thirteen times as compared with the undeformed section.

I claim:

1. A method of making atuning fork comprising the steps of forming a primary bend in the middle portion of a length of strip material so as to produce a fork having two similar tines joined by the primary bend, and deforming the central portion of the primary bend by forming therein at least one secondary bend whose axis extends in a direction substantially at right angles to the axis of the primary bend.

2. A method as claimed in claim 1 comprising the further step of attaching a support member to the centre of the deformed portion.

3. A tuning fork comprising a length of strip material having a central primary bend extending in the direction of the length of the strip to form two tines joined by the primary bend, and at least one secondary bend, at least one of the axes of the secondary bend at the center of the primary bend extending in a direction substantially at right angles to the axis of the primary bend, the secondary bend being so placed that a region of the material of the strip which is unstressed when the fork is oscillating is substantially co-extensive with a portion of one surface of the strip.

4. A tuning fork as claimed in claim 3 comprising a support attached to the said co-extensive portion of the said one surface of the fork.

5. A tuning fork as claimed in claim 3 in which the secondary bend extends outwardly of the fork.

6. A tuning fork as claimed in claim 3 in which the secondary bend extends inwardly of the fork.

References Cited UNITED STATES PATENTS 2,994,241 8/1961 Gibbs 84-409 3,167,905 2/1965 Hetzel 58-23 3,257,794 6/1966 Bennett et a1 58-23 3,303,705 2/1967 Dostal 84-409 RICHARD B. WILKINSON, Primary Examiner E. C. SIMMONS, Assistant Examiner US. Cl. X.R. 

